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Finding the size of Beam (cross members/knee braces)

  1. Oct 22, 2013 #1
    Please find the attached picture. It clearly explains the problem

    What is the method to find the size of the knee braced beam member?

    Do we have to to draw free body diagram for each point and then find out the force acting on the cross member-then see if that beam's cross section area can withstand the derived force?

    is (33069 # /2 )* Cos 45 the force on each cross member beam ?

    Attached Files:

  2. jcsd
  3. Oct 23, 2013 #2


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    Why, yes you do have to find the forces in each member in order to design it.

    IMO, your frame work is deficient in several respects for good design:

    1. The diagonal braces at the ends should run the full depth of the frame, i.e. they should start in the bottom corners and run to the top cross piece.

    2. Having a large load land directly in the middle of a beam is asking for trouble. At the very least, there should be a vertical member directly beneath the load point to help the load spread into the top and bottom of the frame.

    3. I would alter the current arrangement of diagonals at the end and make a true truss structure, like a Pratt truss:


    The ends of the truss don't necessarily have to be sloped like this image; the vertical ends are OK.

    4. There are various codes which you can use to design the truss, such as AISC. If you have no experience whatsoever at steel design, I would recommend that you hire an engineer. At least that way, you can get all of the details of the connections straight the first time, and get your members sized properly. Who knows, you might save some steel, or at least put it where it can do you the most good.
  4. Oct 23, 2013 #3
    This may not be good engineering design, but in some circumstances, it might qualify as good architectural design. If you really wanted to achieve that shape, you should make sure the four enveloping members are continuous, and have other members in place not shown in the drawing that prevent out-of-plane buckling. That said, you would have a redundant structure and you should use statically indeterminate analysis to determine forces AND moments in all the frame members, not just the diagonals. This may be beyond you at the present time. You would need to establish whether the joints are to be pinned or rigid. All that is perhaps going beyond your question, but there is a beam at the library of Durham cathedral in UK where a timber beam is supported by intermediate diagonals, and result is at least reduced deflection of AB. In that particular case, there is no member EF, except massive masonry at E and at F.
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